The switch of the present invention consists of a pair of leads which have contact portions thereon that are initially in a closed circuit condition. The leads are held together by a pellet formed of an organic-glass bead mixture which melts when the ambient temperature exceeds the melting temperature of the organic material in the pellet, thereby allowing the contacts to spring apart to open the circuit. A related type of thermal switch is shown in U.S. patent application Ser. No. 844,414 filed Oct. 21, 1977, now U.S. Pat. No. 4,135,176 in the names of John Klose McVey, Bruce Arthur Luxon, and Larry Lee Sharp and assigned to the assignee of the present invention. Like the thermal cut-off device of the present invention, the contacts of the thermal switch of the McVey et al. application are in a normally closed position and one of the contacts has a ball-shaped projection on it.
However, in the present invention the second contact has a socket-shaped indentation on it which receives the ball-shaped projection on the other lead, while in the McVey et al. application, the ball-shaped projection merely contacted the flat surface of the contacting lead. The improved ball and socket connection of the thermal switch of the present invention allows for appreciably greater current capacity, lower temperature rise due to current through the leads and easier assembly of the leads in the device since they tend to lock together during insertion of the leads in the housing of the device, because of the ball and socket arrangement.
Also, in the prior thermal switch of the McVey et al. application a cement layer was also used to seal the bottom portion of the thermal switch, and a pellet of organic material was inserted into the device so that it was separated from the walls of the outer casing by some of the conductive sealing cement. The organic pellet of the device of the prior McVey et al. application, however, like numerous other prior cut-off pellets, was constructed of an organic material that was pressed together to form a coherent mass. Pressing of an organic pellet, results in a structure which has voids in it, and the pellet is structurally relatively weak and subject to cracking or breaking. The present invention also provides a thermal switch which has a much stronger pellet, is less costly because less organic material is required, and is easier to manufacture.